That's a lot of vintage vibe.
Sometimes it takes a little extra work to get a neck straightened out. Especially on one of these things. This is a 60's Conrad Bass made in Japan and has many cousins probably made in the same factory with names like Lyle, Conquistador, etc etc. They are pretty neat but can have some problems. Jeff took the truss rod nut out, cleaned it up, lubed it, and installed a nylon washer so that the nut could get a few more turns, before the threads bottom out. Even with all that he needed to straighten the neck a tiny bit. So as you see below, using a stiff aluminum rail, with some clamps and cauls, he clamped it into a back-bow so he could put a few more turns on the rod without applying too much force and stripping it. It did the trick and he had it setup and playing great later that day. A word of warning to the DIY player who works on his instruments at home: If you have any doubts while adjusting a truss rod, STOP, and take it to a professional. If the truss rod nut becomes stripped it can be very hard to remove or adjust. If the truss rod breaks, you may be looking at a very expensive repair that in many cases is more than the instrument is worth. So be careful!
Clamped for adustment